Producing viaholes in plastic sheets and application of the method

ABSTRACT

Viaholes are dry-etched into glass fiber reinforced plastic sheets according to a predetermined hole pattern, leaving the glass fiber meshing practically unaffected. 
     The method is used, for instance, to fabricate plastic sheets with unilaterally or bilaterally applied conductor patterns and viaconnectors that are conductively linked to the conductor patterns, and to fabricate multilayer board laminates obtained by several plastic sheets carrying the conductor pattern being alternately packaged, if required, with untreated copper sheets, and by the package thus obtained being subsequently laminated. Such boards and/or plastic sheets bilaterally provided with conductor patterns may be used as connector boards for, say, multilayer ceramic modules carrying semiconductor chips.

This is a divisional of co-pending patent application Ser. No. 742,684,filed on June 10, 1985.

BACKGROUND OF THE INVENTION

The present invention concerns a method of producing viaholes in glassfiber reinforced plastic sheets and an application of that method forfabricating plastic sheets with unilaterally or bilaterally appliedconductor patterns and viaconnectors conductively linked to theconductor patterns.

The method is used in particular to fabricate multilayer boards. Suchboards are laminates of several glass fiber reinforced plastic sheets,to every other one of which a conductor pattern is unilaterally orbilaterally applied. The conductor patterns are conductivelyinterconnected by copper layers on the walls of viaholes in the plasticsheets. The methods suitable for fabricating multilayer boards include,for instance, the so-called "pin-parallel method". This method isdescribed in the article "High-density board fabrication techniques" byJ. R. Bub et al published in the IBM Journal of Research andDevelopment, Vol. 26, No. 3, May 1982, p. 306 and subsequent pages. Bythis method a copper layer is bilaterally applied to a plastic sheet.Subsequently, holes in accordance with the desired placement of theviaholes are drilled into the copper layers and the plastic sheet eithermechanically or by a laser beam. The holes are cleaned and activated forelectroless plating by means of a solution produced from PdCl₂ andSnCl₂. The two copper layers are subsequently covered with a photoresistmask, representing the negative of the desired conductor pattern, andfinally copper is deposited in the holes and on the bared regions of thecopper layers. Then the photoresist masks and the bared regions of thethin copper layer are removed. From the plastic sheets thus obtained apackage is formed, alternately using untreated plastic sheets, andlaminated. Finally, throughholes are drilled into the package and, afteractivation (see above) their walls are electrolessly copper-plated.During the formation of the holes with a diameter of about 0.25 mm inthe about 0.15 mm thick and about 70×60 cm large sheets and also inparticular during the cleaning of the holes, for example, bysandblasting, the plastic sheets are subjected to high thermal and/ormechanical stresses. The holes have a very small spacing, so that closetolerances have to be observed during drilling or laser beam treatment,which is difficult to accomplish by the tools described. Apart fromthis, the drilling or laser beam irradiation step is time-consuming andexpensive from an apparatus point of view. Furthermore, the mechanicalstability of the plastic sheets is greatly reduced during the formationof the holes, because the glass fiber meshing in the holes is removed inthe same process. Finally, the conductive connectors in the holes formedby the described process fall short of the optimum values, since thecopper coating is thin and not continuous throughout.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a method,by means of which holes in glass fiber reinforced plastic sheets can berapidly, gently and inexpensively formed with tight tolerances, and anapplication of that method during the fabrication of plastic sheets withunilaterally or bilaterally applied conductor patterns andviaconnectors.

This and other objects of the present invention are accomplished bymeans of a method of fabricating a pattern of viaholes in glass fiberreinforced plastic sheets comprising the step of dry-etching theviaholes, attacking only the plastic but not the glass fiber meshing. Anapplication of this method for fabricating a pattern of viaholes in aglass fiber reinforced plastic sheets with unilaterally or bilaterallyapplied conductor patterns and via connectors that are conductivelylinked to the conductor patterns, comprises etching holes into a glassfiber reinforced plastic sheet unilaterally or bilaterally provided witha copper mask. Then, the walls of the holes and the glass fiber meshingare activated with a solution containing colloidal palladium. Next,photoresist masks, representing the negatives of the desired conductorpatterns, are generated on the copper masks and copper is electrolesslydeposited on the bared regions of the copper mask(s), the activatedwalls of the holes and the activated glass fiber meshing. Finally, thephotoresist mask and the subjacent regions of the copper mask(s) areremoved.

Although the glass fiber meshing is practically not attacked during dryetching, the plastic material in the region of the holes is removed socleanly that subsequent cleaning of the holes is not required. Incontrast to known methods, the method in accordance with the inventionensures that all of the holes to be formed in a plastic sheet--an about70×60 cm sheet has some 4000 holes--are simultaneously etched. There arealready dry etching devices on the market that permit several suchplastic sheets to be simultaneously etched. As these dry etching devicesare less expensive than those used for drilling or laser beamirradiation and also afford a higher throughput, the method according tothe invention is far more economical than known ones. In addition, itpermits etching with very small tolerances, so that holes with adiameter of less than 0.25 mm can be produced with a high surfacedensity. As the method according to the invention does not subject theplastic sheets to mechanical stress during or after etching, because, incontrast to known methods, a cleaning step is not necessary after theholes have been formed, it is also possible to etch very thin plasticsheets without any distortions. An additional advantage is that theglass fiber meshing remains unaffected during the formation of theholes, so that there is practically no deterioration of the mechanicalstability of the plastic sheets when the holes are formed.

It is advantageous to generate the holes by plasma etching in anatmosphere containing CF₄ and O₂. For that purpose, it is favourable toetch at an HF power of between about 3 and about 6 kW and a pressure ofbetween about 0.1 and 1 mbar, using a gas flow of 5 standard liters perminute (SLM), in which the proportion of CF₄ ranges from about 25 toabout 45 and the proportion of O₂ from about 75 to about 55 volumepercent.

Therefore, the method according to the invention is particularlysuitable for the fabrication of plastic sheets with unilaterally orbilaterally applied conductor patterns and viaconnectors, since itsadvantages favourably influence the efficiency, accuracy andreproducibility of the fabrication process. The method according to theinvention has the particular advantage that it does not remove the glassfiber meshing in the holes. As during the treatment with the solutioncontaining palladium and tin chloride the glass fiber meshing is alsoactivated, copper plating causes copper to be deposited on the holewalls as well as on the glass fiber meshing, so that the entire hole ispractically filled with copper. As a result, the mechanical stability ofthe plastic sheet is further increased, the conductivity of theviaconnectors is improved over that afforded by known methods, and thesurface in the area of the holes is practically coplanar with thesurface of the plastic sheet, which has the favourable effect that whenseveral such plastic sheets are laminated, they are positioned flatlyand practically failure-free on top of each other.

BRIEF DESCRIPTION OF THE DRAWING

Other advantageous embodiments of the method according to the inventionand its application may be seen from the following detailed descriptionin conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic cross-sectional view of a structure that isassumed as a basis for the method according to the invention,

FIG. 2A is a schematic cross-sectional view of a structure obtained bythe method according to the invention,

FIG. 2B is a schematic cross-sectional view of the structure obtained byprior art methods after the viaholes have been produced, and

FIGS. 3 and 4 are also cross-sectional views of the result of two methodsteps during the further treatment of a structure produced according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The method in accordance with the invention is described in detail belowwith reference to FIGS. 1 and 2A. As described, it is implemented suchthat conductor patterns are generated on either side of the plasticsheet, and the applications described also proceed from such plasticsheets. It is pointed out that the method according to the inventionalso works with unilaterally applied or no conductor patterns, althoughin the latter case it should be difficult to find a suitable applicationin practice.

It is assumed that there is a plastic sheet 1 which is reinforced with aglass fiber meshing 2, consisting of a polymer, such as an epoxy resin,and which is, for example, 70×60 cm large and 0.1 to 0.2 mm thick. Theepoxy resin used may be, for instance, an epoxy resin such as thatmarketed by Ciba-Geigy under the trade name of Araldid 9302. The glassfiber meshing consists of a glass that is poor in alkali. To either sideof the plastic sheet 1, a thin layer 3 of a mask material, say, copper,is applied. For that purpose, a strippable foil, which essentiallyconsists of a 5 m and a 70 m thick copper foil, is laminated to theplastic sheet such that the thinner copper foil, which, in addition, hasa rough surface, is laminated to the two surfaces of the plastic sheet.Subsequently, a hole pattern 4, corresponding to the desired viaholepattern in the plastic sheet, is photolithographically generated eitherin only one or both mask layers 3. This may be done, for example, byapplying a photoresist foil to both sides of the mask layers 3 and bysubsequently unilaterally or bilaterally irradiating and developing thefoil to obtain an image suiting the desired conductor pattern. Thephotoresist foil used may be a foil such as that marketed by DuPontunder the trade name of Riston. If Riston is used, 1,1,1-trichloroethaneis employed for the development step. The bared regions of the masklayer 3 are then removed. If the mask layer 3 is made of copper, this isetched off, for example, by an ammoniacal CuCl₂ or a sulfuric acidicsodium peroxodisulfate solution. Then the photoresist is removed, forwhich purpose, in the case of Riston, methylene chloride is best used. Across-sectional view of the structure existing at that stage is shown inFIG. 1 which assumes that the hole pattern 4 is generated bilaterally.At a size of the plastic sheet of 70×60 cm and a hole diameter of about0.25 mm, the hole pattern 4 consists of 4000 holes.

In the subsequent method step, holes 5 in the regions of the plasticsheet 1 not covered by the mask layer(s) are dry-etched either from onlyone--as shown in FIG. 2A--or from both surfaces. For dry etching, aplasma etch process is preferably used which is implemented in anatmosphere containing CF₄ and O₂ as reactive gases.

For plasma etching, devices are generally used wherein the workpiece tobe etched is located in an evacuated chamber between a cathode-anodeelectrode pair, with the cathode being connected to high-frequency, theanode to ground potential, and the workpiece to a varying potential. Thereactive gas is directed through the chamber. The etch effect of plasmaetching is predominantly chemical. In the plasma produced in thereactive gas between the electrodes reactive ions are generated whichform volatile reaction products with the material of the plastic sheet,thus causing same to be removed. Compared with this chemical effect ofthe ions, a removal resulting from their kinetic energy is scarcely ofany significance. For plasma etching, a system may be used in which theelectrodes are aligned parallel to each other and which is marketed byBranson under the type number 7411. This system comprises ten cells,each of which is surrounded by one cathode-anode pair, and in which oneplastic sheet measuring 70×60 cm is processed at a time. That means thatten plastic sheets of that size can be simultaneously etched. Duringetching, the plastic sheets are supported in aluminium stands betweenthe electrodes. The stands and thus the plastic sheets are electricallyinsulated from the high-frequency voltage and ground potential. By meansof a pump, the etching gas is pumped through the cells at a maximumspeed of 900 m³ per hour.

For etching the viaholes in the plastic sheets, a high-frequency ofbetween about 3 and about 6 kW (13.56 MHz), a pressure of between about0.1 and 1 mbar, and a total gas flow of between 3 and 7 standard litersper minute are used, with the proportion of CF₄ in the gas ranging fromabout 25 to about 45 and the proportion of O₂ ranging from about 75 toabout 55 volume percent. During the implementation of the method, theglass fiber meshing is practically not attacked, while the epoxy isremoved without residues or deposits. The schematic cross-sectional viewin FIG. 2A shows the structure after dry etching. Etching leads to theformation of the viaholes 5 in which the glass fiber meshing 2 stillexists in its entirety.

The schematic cross-sectional view of FIG. 2B serves to illustrate whatbecomes of the structure in FIG. 1 if the known methods of forming theviaholes, i.e., drilling or laser beam irradiation, are applied. Themain difference between that structure and the one shown in FIG. 2A isthat there is no glass fiber meshing 2 left in the hole 5 shown in FIG.2B.

Subsequently, the method according to the invention will be describedstill further with reference to two examples. In both examples, about4000 holes with a diameter of 0.25 mm were etched in a Branson systeminto five expoxy resin sheets with a size of 70×60 cm and a thickness of0.15 mm. The conditions were as follows:

high-frequency power: 4 kW

frequency: 13.56 MHz

pressure: 0.6 mbar

total gas flow: 5 SLPM

CF₄ content in gas 35 volume percent

O₂ content in gas, 65 volume percent

In both examples, a 5 m thick copper foil was bilaterally laminated tothe plastic sheets. The examples differed in that in the event ofexample 1, only one of the copper foils was provided with a hole patternaccording to the desired viahole pattern, whereas in the event ofexample 2, both copper foils of the sheets used were provided with sucha pattern. Under the described conditions, the etching speed for theexpoxy was about 2.5 m per minute. In the event of example 1, theetching time was about 65 minutes and in the event of example 2 about 32minutes. It was found that the glass fiber meshing remains practicallyunaffected by the etch process. The tolerances of the dimensions of allholes etched in either example were small. Even in the event of example1, where etching was effected from only one surface of the plasticsheet, the epoxy resin was satisfactorily etched to the opposite copperfoil. No particulate or greasy deposits were found in the etched holes.

The method in accordance with the invention may be advantageouslycombined with the method of fabricating plastic sheets with unilaterallyor bilaterally applied conductor patterns and viaconnectors that areconductively linked to the conductor patterns. The sheets thusfabricated are used either in that form or, for example, after a packagehas been formed of several of them and, if required, of interposeduntreated plastic sheets, and the package thus obtained has beenlaminated and provided with throughholes, as an electrical carrier forceramic modules to which semiconductor chips with integrated circuitsare soldered. A prerequisite for applying the method in accordance withthe invention in that way is that it is possible to produce connectorswith a good conductivity in the holes formed by it. That prerequisite isnot only met by the method in accordance with the invention but theresult achieved by it is superior to that of known methods. While withknown methods the holes formed have to be cleaned first before they canbe activated for copper plating, such cleaning is eliminated with thenew method, and during activation with a palladium/tin chloride solution(the finished solution containing colloidal zerovalent palladium), themethod according to the invention, unlike known methods where there isno glass fiber material left in the holes, causes the hole wall and theglass fiber meshing to be activated. As a result, during the subsequentelectroless deposition step, copper is deposited both on the hole wallsand the glass fiber meshing, thus ensuring--as previously described--notonly an improved electrical connection but also an increase inmechanical stability.

The entire process of fabricating plastic sheets with bilaterallyapplied conductor patterns and conductive connectors for linking theconductor pattern, utilizing the method according to the invention, ispreferably carried out in the manner described by way of FIGS. 1, 2A, 3and 4:

1. To an about 70×60 cm and 0.15 cm thick copper sheet 1 of glass fiberreinforced epoxy resin a strippable foil consisting of a thicker (70 m)and a thinner (5 m) foil is bilaterally laminated such that the surfaceof the thinner copper foil, which is relatively rough, rests against theplastic surface. The thin copper foil serves as a mask layer 3.

2. The thicker copper foil is stripped from either side.

3. This is followed by the method according to the invention, as haspreviously been described in detail. That means that first of all thehole pattern 4 is photolithographically generated in the 5 m thickcopper foils 3 (the resultant structure being shown in the schematiccross-sectional view of FIG. 1) and that the viaholes 5 are subsequentlyplasma-etched, using the copper foils 3 as etch masks. The resultantstructure is shown in FIG. 2A.

4. The walls of the viaholes 5 and the bared glass fiber meshing 2 arethen activated for electroless copper plating in a solution obtained bydissolving palladium and tin chloride in a diluted hydrochloric acidsolution and containing colloidal zerovalent palladium. The resultantstructure is shown in the schematic cross-sectional view of FIG. 3,reference number 6 denoting the activation applied.

5. A photoresist foil consisting, for example, of Riston, is thenapplied to either side of the copper foils 3. Subsequently, a holepattern corresponding to the desired conductor pattern and in which theviaholes 5 have been bared is formed by imagewise exposure and developedwith 1,1,1-trichloro ethane.

6. On to the bared regions of the copper foil 3 and the activatedregions in the viaholes 5 copper is electrolessly deposited by immersionin an alkaline solution containing copper sulfate, EDTA dianhydride, awetting agent, formaldehyde and a small amount of sodium cyanide. Asection of the resultant structure is shown in the cross-sectional viewin FIG. 4, the photoresist mask being designated as 7 and the depositedcopper as 8.

7. In the last method step, the photoresist mask 7 is removed bymethylene chloride, thus completing the plastic sheet with bilaterallyapplied conductor patterns and viaconnectors for connecting theconductor patterns.

If a board with several levels of conductor patterns is to befabricated, the process has to be continued.

8. Several sheets fabricated in method steps 1 to 7 are alternatelypackaged with suitably aligned untreated plastic sheets of the same sizeand about the same thickness and laminated. Then throughholes aredrilled through the package, and subsequently the hole walls areactivated and electrolessly copper-plated.

Boards thus fabricated generally have a thickness of 5 mm and the numberof plastic sheets with bilaterally applied conductor patterns theycomprise is of the order of 15. These boards may be provided withmultilayer ceramic modules comprising a corresponding number ofconductor planes and to each of which a large number of semiconductorchips with large-scale integrated densely packed integrated circuits issoldered.

Of course, the foregoing description is directed to one particularembodiment of the present invention and various modifications and otherembodiments of the present invention will be readily apparent to one ofordinary skill in the art to which the present invention pertains.Therefore, while the present invention has been described in conjunctionwith a particular embodiment, it is to be understood that variousmodifications and other embodiments of the present invention may be madewithout departing from the scope of the present invention as describedherein and as claimed in the appended claims.

What is claimed is:
 1. A method for fabricating glass fiber reinforcedplastic sheets with applied conductor patterns and via connectors thatare conductively linked to the conductor patterns comprising the stepsof:providing a copper mask on a glass fiber reinforced plastic sheet;dry-etching holes into said glass fiber reinforced plastic sheet withsaid copper mask while leaving the glass fiber in the holessubstantially unaffected; activating the walls of the holes and glassfiber in the holes with a composition containing palladium; generating aphotoresist mask, representing the negative of a desired conductorpatterns, on the copper mask so as to leave exposed regions of thecopper mask; electrolessly depositing copper on the exposed regions ofthe copper mask, the walls of the holes that have been activated, andthe glass fiber that has been activated; and removing the photoresistmask and regions of the copper mask lying directly beneath thephotoresist mask.
 2. The method of claim 1 wherein said copper mask isunilaterally applied.
 3. The method of claim 1 wherein said copper maskis bilaterally applied.
 4. The method of claim 1 which further comprisesproviding an assembly of a plurality of said glass fiber reinforcedplastic sheets with applied conductor patterns wherein on some of saidplastic sheets the conductor patterns are unilaterally applied, and onsome others of said plastic sheets the conductor patterns arebilaterally applied; and arranging said assembly so that said plasticsheets with unilaterally applied conductor patterns are packaged inalternating order with bilaterally applied conductor patterns;laminatingtogether the plastic sheets of the assembly; drilling through-holesthrough the assembly after said laminating; cleaning the walls of thethrough-holes after said drilling; activating the walls of saidthrough-holes for subsequent electroless plating thereon; andelectrolessly plating said through-holes with copper.
 5. A methodaccording to claim 1 wherein the step of dry-etching comprises:plasmaetching the viaholes in an atmosphere containing CF₄ and O₂.
 6. A methodaccording to claim 5 wherein the plasma etching is effected at an HFpower of between 3 and about 6 kW and a pressure of between about 0.1and about 1 mbar using a gas flow of between about 3 and about 7standard liters per minute with the gas mixture containing between about25 and 45 volume percent CF₄ and between about 75 and about 55 volumepercent O₂.
 7. A method according to claim 5 wherein the plasma etchingis effected at an HF power of about 4 kW and a pressure of about 0.6mbar using a gas flow of about 5 standard liters per minute with the gasmixture containing about 35 volume percent CF₄ and about 65 volumepercent O₂.
 8. A method according to claim 1 wherein the dry-etchingeffected from one or simultaneously both surfaces of the plastic sheet.9. A method according to claim 1 wherein the plastic sheet which isetched comprises an epoxy resin.